Overall, the movie is okay, but I think Hex's origin story should have been fleshed out in detail. The entire movie clocks in around 80 minutes and that story is probably given less than 20 minutes. Aside from knowing who the good guy and bad guy are, we have no real reason to be invested in any of the characters. This is a shame, given that the supporting cast is made up of some very good actors (Michael Fassbender, Lance Reddick, etc). Megan Fox was pretty much the same as always. She looks pretty and has a couple good one-liners, but her character is not in the movie all that much until the end. And again, there is no backstory for her character so the only reason we are supposed to care about her character is she is pretty and loves Jonah Hex. I think the movie would have been better served to have been a little longer, with the first half giving a true backstory of the events that lead up to the start of this movie, then Hex going after Turnbull initially, and then his transition to a bounty hunter. If that movie did well, then a better version of this movie could have been made and with characters that people actually are invested in, they may have been able to get at least a couple movies from the story.
I have never read the comics, but from reading other reviews, it seems like the filmmakers diverged a lot from the source material. If you are a fan of the comics and changing the story would piss you off, then I would say skip this entirely. For everyone else, the movie is entertaining enough. It could have been better, but with the short running time (especially for comic book movies) it gets in and out pretty quickly.